The pollution of aqueous effluent by waste containing heavy metals represents a significant fraction of the harm due to industry. It is estimated that 30% of industrial pollution is due to the surface treatment sector on its own. As a result, environmental constraints on this sector of activity are becoming more and more severe.
Numerous depollution techniques have been developed: precipitation-filtration; treatment by means of ion-exchanging resins; cementation; reverse osmosis; and electroplating. Particular use has been made of electroplating.
Only a few years ago, the majority of electrochemical reactors made use of electrodes in the form of solid plates suitable for working in media that are highly concentrated in the metal to be electroplated. However, such electrodes are not suitable for obtaining a concentration at the end of treatment compatible with waste standards now in force. Such electrodes are generally used either for preparing various metals by electrosynthesis, or else in the treatment of industrial effluent for the purpose of changing the concentration of an effluent from a high value (typically of the order of several tens of grams/liter (g/l)) to a value making it possible to use new cathodes that are adapted to operating in a dilute solution of electroactive metal.
Progress in electrochemical engineering has contributed to developing systems that use "volume" cathodes which are particularly suitable for recovering compounds that are present at low concentration in the electrolyte. The advantage of such a cathode is to be able to provide a large electrode area per unit volume, and also to provide high speeds of material transfer.
U.S. Pat. No. 4,226,685, in particular, describes a reactor for treating aqueous effluent containing a heavy metal by electroplating, said reactor comprising a tank in which alternating electrodes of opposite polarity are placed, with the cathodes being removable volume electrodes constituted by respective cassettes each comprising two facing frames and each supporting a wall that is permeable to said aqueous effluent, the permeable walls of any one cathode defining a volume that is filled with an active granular material and within which a metal current-feed grid is disposed.
Volume cathodes may be of multiple shapes and may implement multiple technologies. For example there exist cathodes that are porous, fluidized, pumped, etc., each having specific advantages that generally amount to optimum operation in a dedicated range of concentrations.
The main difficulty encountered with such electrodes is that once they are saturated with metal, it is necessary to change the volume electrodes, and in most cases such a change means that the electrolyzer is disassembled, thereby reducing the effectiveness of the method because the electroplating operation is interrupted. Depending on the technology used, this reconditioning of the electrochemical reactor can be quite lengthly. Furthermore, the cathode or the recovered active granular material must be retreated so as to recycle the metal that has been extracted from the industrial effluent. In most cases, treatment takes place in a location external to the site on which the effluent is treated by electroplating.
Thus, the technical problem to be solved by the present invention is to implement an electrochemical installation for treating aqueous effluent containing a heavy metal, the installation comprising a heavy metal electroplating reactor of the type described above with reference to U.S. Pat. No. 4,226,685, in which reactor the removable volume cathodes can be regenerated easily.